So-called mosaic or dot matrix printing devices are used for producing printed images or characters that are formed in the manner of a mosaic from a plurality of individual dots. Such printing devices may, by way of example, advantageously utilize or incorporate an ink printing mechanism which includes an ink print head operating either by piezoelectric or by thermoelectric principles. A characteristic of such ink printing devices is that the character and/or graphic image pattern to be printed on the print medium or substrate is not printed as a single, unitary or continuous form or entity but, rather, is made up of a multitude of individual small dots. The print quality of the resulting characters understandably depends substantially on the resolution which, in turn, is determined by the number of individual dots that make up each character or, in other words, by the quantity of lines and columns of dot positions defined in the character matrix within which the character is formed.
German publication DE 28 43 064 describes an ink print head for use in an ink matrix writing device provided with a multitude of outlet nozzles in a highly dense arrangement and from which individual ink droplets are ejected and guidedly thrown against a print medium. The ejection of droplets in such so-called drop-on-demand or ink jet print heads is effected under the influence of thermoelectric converter elements that are implemented in the form of heat resistors on a substrate, can be individually controlled and are associated with individual ink ducts. When the individual heat elements are triggered by electrical pulses or the like, an ink steam bubble is produced in the respective ink duct and leads to the ejection of an individual droplet of ink. This process, known as the bubble jet technique, makes it possible to construct an ink writing device of high resolution in which the writing or printing head may be fabricated with numerous narrow nozzle openings arranged closely adjacent one to another.
Such an ink print head can be used for black-and-white as well as for color ink printing. In a multiple-color printing apparatus, the individual ink print head may have or include a plurality of partial ink print heads associated therewith and in which each partial ink print head is separately and independently operable for impressing or ejecting a separate color ink. When using the primary colors cyan, magenta and yellow, a total of eight different colors can thereby be produced at one pixel position--i.e. white (no printing); cyan, magenta and yellow when only one primary color is printed; red, green, and blue when two primary colors are printed one atop the other; and black by printing all three primary colors in superposed relation atop one another. A black pixel can also, or alternatively, be produced by directly printing an individual pixel with black ink instead of the superimposed printing of cyan, magenta and yellow ink dots. This latter alternative has the advantage that a notably decreased or "simple" amount of solvent is applied to the paper or print medium since only one pixel is printed rather than three, or one drop of ink is applied rather than three. Moreover, the cyan, magenta and yellow inks are not so ideal that their superposed application to the print medium will result in a pure black. This alternative procedure of printing the color black is further advantageous since it necessitates the use of only the black print head when the device or apparatus is operated as a black-and-white printer.
The overall color impression produced by color printing is created by additive color mixing of the individual pixels of the aforementioned eight colors applied adjacent one another on the print medium.
When color printing is carried out with such an ink printing device, the increased application of color can result in a running or blurring of adjacent pixels which are not yet dry. This leads to a washed-out graphic image and is disadvantageous for achieving edge sharpness of the character or image thereby produced. There is also the further risk that the print medium will become uneven or wavy as a result of the excessive moisture of the ink liquids that are applied to the medium.
In addition, it is often also possible to print on different types of print media with such ink printing devices such, for example, as individual sheets, margin-perforated endless paper webs or transparent sheets as used, e.g., in overhead projectors and the like.
A further problem occurs when printing on transparent, smooth sheets at full dot density (e.g. 300 dpi), which often results in running of the applied liquid ink and of the printed image impressed on or to the sheet of print medium material as the individual ink droplets run together when making contact with the sheet.